Conventional rubber products such as tires contain epoxidized natural rubber. Typical epoxidized natural rubber is produced through a series of steps of: concentrating field latex collected from rubber trees (e.g. Hevea brasiliensis) by centrifugation; adding a surfactant to the resulting concentrated latex and subsequently adding formic acid while stirring; slowly introducing hydrogen peroxide over several hours and then allowing the epoxidation reaction to proceed for about one day; coagulating the resulting epoxidized natural rubber in latex form; and optionally neutralizing, water washing, and drying the coagulated natural rubber latex.
The series of production steps advantageously allow uniform epoxidation of rubber because the rubber is epoxidized while having the same particle size (0.1 to several μm) as when it is present in latex. Unfortunately, however, the production cost of epoxidized natural rubber is very high due to the long reaction time, the use of expensive chemical agents, the large number of steps, and the like. Moreover, the rubber is destabilized such that it can easily coagulate; therefore, a surfactant needs to be added, which increases the cost and also creates other problems: for example, a reduction in the rubber physical properties of a final rubber product due to the absorption of moisture caused by the surfactant remaining in the product; and difficulties in controlling the temperature during the epoxidation reaction, which require the operator to continuously monitor the reaction.
Meanwhile, Patent Literature 1 discloses a method for producing an epoxidized natural rubber latex by mixing natural rubber latex with peracetic acid prepared from hydrogen peroxide and acetic acid and/or acetic anhydride. This method allows a rapid and quantitative chemical reaction, so that a desired degree of epoxidation can be rapidly achieved. In this method, however, since the reaction between hydrogen peroxide and acetic acid and/or acetic anhydride is in equilibrium, excess amounts of acetic acid and hydrogen peroxide are required to obtain a necessary amount of peracetic acid. Thus, the method has problems that, for example, it needs excessive amounts of expensive chemical agents, as well as an extra cost for treating wastewater containing the excessive chemical agents.